docs: def export_by_tag_csv(ekey, dstore):
      """
      :param ekey: export key, i.e. a pair (datastore key, fmt)
      :param dstore: datastore object
      """
      token, tag = ekey[0].split('/')
      data = extract(dstore, token + '/' + tag)
      fnames = []
      writer = writers.CsvWriter(fmt=writers.FIVEDIGITS)
      for stat, arr in data:
          tup = (ekey[0].replace('/', '-'), stat, ekey[1])
          path = '%s-%s.%s' % tup
          fname = dstore.export_path(path)
          writer.save(arr, fname)
          fnames.append(fname)
      return fnames